Criticals and Fumbles

If a player rolls a natural 20 on an attack or save, then they have rolled a critical. A critical is not an automatic success, but is considered to be 10 higher, granting a 30 before modifiers. Similarly, rolling a natural 1 is not an automatic failure, but is considered to be 10 lower, granting a -9 before modifiers.

Attacks

Critical Hit

If an attack hits and the roll is either a natural 20 or within the threat range of the attack used before modifiers, then it is considered a critical threat. To confirm the critical, you roll another attack roll with the same modifiers as the roll you just made. If the second attack roll is also a hit, then the attack is a critical hit. If the confirmation misses, then resolve the hit normally.
A successful critical multiplies the damage of your attack by default. Any static modifiers to your damage are multiplied, and the amount of dice you roll for the hit are multiplied. Unless otherwise specified, the multiplier for a critical hit is x2. Any damage dice added to a weapon's normal damage (such as from sneak attack or a flaming enchantment) are not multiplied from a critical hit. Static increases (such as a +2 enhancement or a paladin's smite evil) are multiplied.
A player may elect to forgo the critical damage from an attack to instead add a critical effect to the hit. If so, a location die is rolled to determine where the critical occurs, along with the appropriate critical die for the location and attack type. The DM may in some cases allow for both critical damage and a critical effect die depending on the situation, and certain enemies may be resistant to critical effects, reducing the potential severity of the rolled outcome.
A spell that requires an attack roll to hit can score a critical hit just like a normal attack. All spells have a crit range of 20 and multiplier of x2 by default. Critical effects cannot be added to a spell attack.

Critical Fumble

If an attack rolls a natural 1, then it is considered a fumble. If it misses, the attacker must roll to confirm the fumble much like a critical. If the confirmation is a hit, then the fumble is averted. If the confirmation is a miss as well, then the attacker rolls the appropriate fumble die to determine the effect of the fumble. A player may spend a Luck Point to automatically avoid a fumble, but they must spend it before they roll to confirm the fumble. Each time a player confirms a fumble, they also obtain a Luck Point.